This invention relates to broadband wireless communication utilizing an ATM backbone and more particularly to a system and method of providing a T1/E1 or fractional T1/E1 synchronization signal over a wireless link.
Local Multipoint Distribution Service (LMDS), known as Local Multipoint Communication Service (LMCS) in Canada, employs microwave frequencies in the 2 GHz to 42 GHz range to deliver broadband services over wireless links. Base transceiver stations (BTS) linked to a broadband network such as an asynchronous transfer mode (ATM) backbone provide a vehicle for transmitting broadband services to customer premise equipment (CPE) at fixed locations within the cell served by the base station. This allows small and medium sized businesses to gain access to broadband services such as voice, video, and data without incurring the costs associated with terrestrial connections such as optical fiber cable, hybrid coax, etc. The wireless link in this context has become known as the xe2x80x98last milexe2x80x99 solution to accessing developing broadband networks.
In the base station, which may be, for example, a Newbridge 36170 multi-service switch, an ATM Radio Interface Card (ARIC) interfaces with an outside transceiver (OTX), usually roof mounted, to convert broadband digital information received from the ATM backbone to a radio frequency (RF) signal for transmission, point to multi-point, for reception by network interface units (NIUs) at the customers"" sites. Each of the NIUs may be linked to a variety of broadband devices at the customer""s premise and to a transceiver for communicating bi-directionally with the base station using a point to point protocol.
Because the microwave transmission is basically line of sight the cell or geographical area serviced by a base station is usually not more than a few miles in diameter. The location of the transceivers at both the base station and the customer""s premises also has a bearing on the quality of the communication and hence, the effectiveness of the system.
ATM is a packet oriented technology employing fixed length cells and is well suited for the transport of bursty data. In order to serve a wide range of applications, however, ATM must also be capable of transporting constant bit rate (CBR) traffic such as voice. This means that both the ATM source and the ATM destination must be in synchronization in order to avoid loss of data due to a frame slip in the signal received at the destination.
In wired or terrestrial digital systems, clock distribution is hierarchical with the highest stability source located at the highest point in the system. This will typically be a stratum 3, or higher level, reference source. As clocking is passed downwards through the network, each node takes its timing from either a locally generated clock source, typically of lower stability, (stratum 4 for example) or from the higher level stratum network reference. When a node is running on its own local reference, controlled slips periodically occur in the data.
To alleviate this problem, clock synchronization is passed between nodes to allow slip free operation to be realized. This can be done by extracting timing from an incoming link interconnected to a higher stability source or xe2x80x98out of bandxe2x80x99 using a dedicated synchronization link to each node.
One method of providing synchronization in a wired network is described in U.S. Pat. No. 5,260,978 which issued Nov. 9, 1993 to Fleischer et al. The technique described in the ""978 patent is known as a Synchronous Residual Time Stamp (SRTS).
In a wireless ATM based network the synchronization requirement still exists but the transfer of a clocking signal over a wireless link can be problematic. This is particularly true when the system supports low bandwidth services such as Nxc3x97DS-0 or fractional T1 or E1 service. For example the aforementioned SRTS technique does not work effectively for structured or fractional T1 and E1 services.
Accordingly, there is a requirement in broadband wireless systems, and in particular systems capable of low bandwidth services, to develop an effective method of providing synchronization for traffic between a base station and a NIU.
Therefore in accordance with a first aspect of the present invention there is provided a method of providing a synchronization signal between a base station and a remote network interface unit (NIU) in a wireless network the method comprising: deriving a reference signal at the base station, the reference signal having a frequency which represents a common denominator of all transmission rates to be carried by the network; locking the symbol rate of data transmitted between the base station and the NIU to an integer multiple of the reference signal; retrieving the reference signal at the NIU by dividing the symbol rate by the multiple integer and utilizing the reference signal as a synchronization signal.
In accordance with a second aspect of the invention there is provided a system for providing a synchronization signal between a network interface unit (NIU) and a base station over a wireless link comprising: clock means at the base station for deriving a reference signal, the reference signal having a frequency representing a common denominator of all transmission rates of data to be transmitted; means at the base station to generate a data symbol rate for data transmitted therefrom, the symbol rate being an integer multiple of the reference signal; division means at the NIU to derive the reference signal from the symbol rate by dividing the symbol rate by the integer multiple; and means to synchronize the NIU to the base station using the reference signal.
The invention will now be described in greater detail with reference to the attached drawings wherein:
FIG. 1 is a high level diagram of a cellular, broadband wireless system;
FIG. 2 illustrates the wireless ATM circuit emulation topology according to the present invention;
FIG. 3 illustrates functional aspects of the base station ATM switch;
FIG. 4 illustrates the wireless ATM circuit emulation topology of FIG. 2 in a network implementation;
FIG. 5 is a circuit diagram of the base station to modulator synchronization generator; and
FIG. 6 is a circuit diagram of the network interface unit synchronization recovery topology.